Coating magnesium articles



atented July 29, 1941 1+ rica ooa'rmc MAGNESIUM narrows Herbert K. De Long, Midland, Mich assignor to The Dow Chemical Company,

Midland, IVIiclL,

a corporation of Michigan 4 No Drawing. Application May 24, 1940,

Serial No. 336,962 7 12 Claims.

This invention relates to an improved method of forming aprotecting coating upon articles of magnesium and its alloys. It also concerns a method of coloring such coatings.

An object of the invention is to provide a method of producing strongly adherent, extremely dense corrosion-resistant coatings on magnesium and its alloys. Another object is to describe such a method in which the coating may be formed in a wide variety of colors.

According to the invention, protective coatings are produced by subjecting a magnesium or magnesium alloy article to the action of an aqueous alkaline solution, the process being carried out under pressure at a. temperature between about 150 C. and about 250 C. Colored coatings may be prepared by adding to the treating solution a dye soluble therein.

In practice the magnesium or magnesium alloy article is first cleaned, as by wire brushing, sandcoatings according to'theinvention are far superior in density and corrosion resistance to coat-' ings prepared by known aqueous or alkaline treat: ments at lower temperatures, and can be produced on any-magnesium-base alloy, rather-than on certain types of alloys only.

blasting, acid pickling, or bufing according to the lustre desired in the ultimate coating. The degree of lustre depends upon the smoothness of the metal surface before being subjected to the coating process. 7 For example, sandblasted metal exhibits a relatively dull coating, while bufied-or polished metal exhibits a 'smooth lustrous surface, after treatment according to the invention. After the cleaning operation, the article is immersed in an aqueous alkaline solution, ata temperature between about 150 and 250 C., the treatment beingcarried out in a closed vessel and under a pressure at least equal to the vapor-pressure of the solution at the operating temperature. The treating solutionis usually made by dissolving in water one or more alkalies, preferably an alkali metal hydroxide, such as sodium, potassium, or lithium hydroxide. Other alkaline agents, such as lime, may sometimesbeemployed, although less satisfactorily. Alkali concentrations in the range of 2 to 20 per cent or more by weight are suitable, with about 10 per cent being preferable,

particularly with sodium hydroxide.

While the process may be conducted at any temperature between 150 and 250 C., temperatures in the range between about 170 and about 210 C. give definitely superior coatings, and optimum results are obtained at-about 200 C. The duration of the treatment may be from 1 to 30 minutes or more, depending'on the temperature, At 200 C. a time of 5 minutes suflices.

By the process described there is, produced upon the magnesium or magnesium,alloy article a coatin: which appears to be integral with the article, is relatively thin, flexible, hard and dense. The

Pontacyl Light Green SF Yellowish 670 Malachite Green 657 Wool Green B 737 Janus Green 133 Aniline Red 677 Methyl Red 211 Pigment Scarlet 3B 216 Fast'Crimson 6BL 57 Eosine Yellow 768 Quinaldine Yellow 801 Indian Yellow G 146 Naphthol Yellow 5 1 1'0 Nitrazine Yellow 640 Methyl Violet 680 Crystal Violet 6B 681 Methylene Blue 2BC [922 Aniline Blue 689 Alizarin Blue S 1067 Eosine Bluish 771' Benzo Blue 23 406 Alizarin Blue G 1066 Fast Acid Black -N23 304 Diazine Black H Extra 401' Niagara Sky Blue 63 518 Acid Blue-Black 246 Diamine Black BH 401 Orange 11 151 Orange G 27 A further advantage of the invention is that the coatings can be obtained in a wide variety of colors simply by including in the aqueous alkaline solution a small proportion ofa dye which is soluble therein and is chemically stable under the conditions of treatment. In general, a quantity -of dye corresponding to between about 0.1 and about 0.5 per cent by weight of the solution is used, depending upon the shade desired. The coating thus obtained is homogeneously colored throughout, so that if the coating becomes scratched, no serious mar or color streak is evident.

The following dyes are illustrative of those suitable for use in the process, the number opposite the dye name being that of the Colour Index. 1st edition, January 1924, published by the Society of Dyers and Colourists, Assurance Buildings,

Bradford, Yorkshire, England:

Dye Color Index No.

The foregoing description is illustrative rather than strictly limitative, the invention being coextensive in scope with the following claims.

This application is a continuation-in-part of my co-pending application, Serial No. 224,510, filed August '12, 1938. v

'I claim:

1. The method of pr'oducing a corrosion-resistant coating upon an article formed of a metal selected from the class consisting'of magnesium and magnesium alloys which comprises subjecting the article to the action of a solution con:-

sisting of water and an alkali at a temperature between about 150 C. and about 250 C. and under a pressure at least equal to the vapor presand under a pressure atleast equal to the vapor Dye Color Index No. pressure of the solution at the operating tempera- Methyl Orange 142 ture. Alizarin Y 20% Paste 1040 4 A process according to claim 3 wherein the Fluoresc n 766 temperature is between about 170 C. and about Rosolic Acid, 724 5 210 C. Bismark Brow 331 5. A process according to claim 3 wherein the Phenolphthalein 764 temperature is about 200 C. A i Fnohsine 30 6. A process according to claim 3 wherein the Azo Fu hsi 6B 154 alkali metal hydroxide is sodium hydroxide. RhOdaminE'R 749 10 7. A' process according to claim 3 wherein the Xylene Cyanol FF 715 alkali metal hydroxide is lithium hydroxide. 'cochineal 1239 8. The method of producing a colored corro- Alizarin Cyanine 1050 sion-resistant coating upon an article formed of B d a R 33 a metal selected from the class consisting of mag- Safranine GR 343 nesium and magnesium alloys whichcomprises Lanafuchsine ma 54 subjecting the article to the action of a solution Dimethyl-amino-azo-benzene (Butter Yelconsistmg O er a d an alkali and a small D 1 1 portion of a dye soluble therein, the treatment being carried out at a temperature between about C. and about 250 C. and under a pressure at least equal to the vapor pressure of the solution at the operating temperature.

9. A process according to claim 8 wherein the alkaline solution is an aqueous solution of an alkali metal hydroxide present'in a. concentration between about 2 and about 20- per cent by weight of the solution, and wherein the treatment is car- I ried out at, a temperature between about C. and about 210 C. y

10. An article formed of a metal selected from the class consisting of magnesium and magnesium alloys and having thereon an adherent corrosion-resistant coating produced according to the process of claim 1.

11. An article formedof a metal selected from the class consisting of magnesium and magnesium alloys and having thereon an adherent corrosionresistant coating produced according to the proces of claim 3. v

12. An article formed of a metal selected from the class consisting of magnesium and magnesium alloys and having thereon a homogeneously colored adherent corrosion-resistant coating produced according to the process of claim 8.

HERBERT K. DE LONG. 

